This invention relates to a method and apparatus for fingerprint verification and more particularly to apparatus for recording a reflection hologram of fingerprint data and to a method and apparatus for optically filtering input fingerprint data with the hologram to form a correlation signal.
It is common to purchase goods and services and obtain cash from Automatic Teller Machines (ATMs) through the use of credit or bank cards. Currently, over 900 million plastic magnetic stripe credit and debit cards are circulating in North America and there is no reliable method to verify that the card user is the legal user. The Personal Identification Number (PIN) in the case of ATM access, or the signature on the invoice form which the card user signs does not constitute nor ensure positive identification. Signatures can be falsified and PINs can be surreptitiously obtained.
The credit card industry in the United States transacted about $350 billion in charge volume in 1986 and the concommitant losses due to fraud was close to $750 million. Stolen cards make up 21 percent of this loss for a total of $150 million. It is known that federal police forces have become deeply involved in trying to stop interstate and even intercontinental schemes to defraud thousands of credit card holders. These developments place existing credit cards in a position of not just having occasional security problems, but of being susceptible to broadscale criminal activities with little or no protection.
Incorporation of a photograph of the legal user on the card is another method used with charge cards; this method is also used with passports. However, this method also does not allow for positive identification. When a system relies on the human element of a guard service, for example, visual identification of an individual by a photograph on an ID card can be impaired by personal stress, fatigue, outside disturbances, or just sheer numbers of people seeking entry. Furthermore, appearances can be altered to match the photo on the card and cards can be forged with the photo of an illegal user.
Overall, signatures, PINs and photographs have served as imperfect parameters for card user verification. Moreover, the fraudulent use of cards will cost companies in North America in excess of one billion dollars in the next year.
With the rising cost of fraud the use of fingerprints as a verification parameter has been explored. Fingerprints are unique to each individual and thereby constitute positive verification.
Optical processing techniques using holographic matched filters have been a major conceptual advance in the fingerprint identification area. Optical processing methods differ fundamentally from digital techniques in that the reference information is manipulated not in the image plane, but in the Fourier transform plane normally formed by a lens. Verification is accomplished by superimposing the "live" fingerprint after Fourier transformation onto the transformed reference fingerprints. This procedure achieves a correlation between the two sets of fingerprint patterns, the result of which is a focused beam of light if a match occurs. This technique is a real time parallel processing method which allows the verification cycle to be completed within a second or so (the time from placement of the user's fingerprint on the read lens to a "match" or "no-match" identification).
Theoretically, more accurate verification can be obtained with optical processing. Fingerprint patterns are succeptible to degradation and damage because of the wear and tear of everyday activity such as cuts, abrasions and foreign contaminants. The wear and tear contributes "noise" to the comparison system and if not compensated will lead to higher false positive and false negative verifications. To compensate for "fingerprint noise" with digital techniques in the real time domain is a complex and expensive process since an algorithm using serial processing would have to be used. In the frequency transform domain, though, the removal of "fingerprint noise" is inherent in the comparison process. Since comparison is accomplished using a holographic matched filter, the matched filter removes all spatial frequencies which are not within the band comprising the reference fingerprint. This turns out to be a major portion of the "noise" spectrum. Furthermore, by the use of an appropriate high pass spatial filter, undesired low frequencies and dc biases can be removed to improve the correlation accuracy. Accordingly, optical processing can lead to lower false positive and false negative verification.
Methods to identify fingerprints using Fourier transform holograms and optical correlation techniques have been described in the art. The practicalities, however, associated with building such devices have been disappointing. Devices have been expensive, complex and unreliable thereby precluding their widespread use in the commercial marketplace.
To date, the methods suffer from a number of disadvantages, foremost among which is a high degree of inaccuracy in a real world environment. For example, mismatch can occur because of rotational misalignment and/or scale changes between the image and hologram. This can result from a slight rotational movement of the finger in the identification device or inconsistent optics between the device that records the hologram and that which identifies the card user. Scale changes also result if the user's fingerprints enlarge as may occur with swelling or obesity. A decrease in the signal-to-noise ratio of the system by a factor of 500 results from only a 3.5 degree change in rotation or a 2 percent change in scale between the "live" and reference fingerprint patterns.
Prior art devices also are dependent on the use of coherent light sources during both the recording and identification cycle which increases the expense and complexity of the devices. Sensitivity is a further problem. The optical system must have positional stability to maintain a high signal-to-noise ratio and can be severely disturbed by a slight vibration which can knock an optical element out of position.
Another difficulty as illustrated by U.S. Pat. No. 3,781,113 to Thomas issued Dec. 25, 1973 and U.S. Pat. No. 3,704,949 to Thomas et al. issued Dec. 5, 1972 is the cumbersome method of measuring the correlation signal. In both of these patents a motorized reticle is used to chop the light in order to distinguish between focused (correlated) and unfocused (uncorrelated) light.